1. Field of the Invention
The present invention relates generally to laser ablation, and more particularly pertains to a system and method for eliminating structure and edge roughness, which is produced during the laser ablation of a material.
Typically, in this particular technology, an ultrashort pulsed laser beam is utilized to ablate undesired extra material, which is present in a photomask. The pulsed laser beam is fired in a programmed spatial pattern, thereby removing the encountered extra material which causes a defect. However, it is possible that the process of laser ablation can produce a periodic structure in the irradiated material, whereby this periodic structure produces a significant roughness at the edge of the ablated defect, which degrades the optical quality of the repaired photomask. Consequently, there is a need to provide a system and method which will prevent or eliminate this edge roughness, and thereby to resultingly produce a repaired photomask with improved optical quality.
2. Discussion of the Prior Art
In the current state-of-the-technology, a number of publications are known which disclose and teach the application of equipment and methods, which are required in order to remove defects encountered in lithographic masks. To that effect, an ulstrashort pulsed laser beam may be utilized to ablate undesired extra material in a programmed spatial pattern, thereby removing the encountered defects. The foregoing aspects are disclosed in Grenon, et al., U.S. Pat. Nos. 6,190,836; 6,165,649; 6,156,461; 6,090,507; and Haight, et al., U.S. Pat. No. 6,333,485.
Furthermore, as known, laser ablation can produce a periodic structure in the irradiated material, thereby resulting in a significant degree of roughness at the edge of the ablated defect, which degrades the optical quality of the repaired photomask. This aspect is discussed in various publications, such as, for instance, the following articles: “Laser Induced Periodic Surface Structure: Experiments on Ge, Si, Al, and Brass”, Young, Preston, vsn Driel, and Sipe, Physical Review B, Vol. 27, No. 2, pgs. 1155-1172 (1983); “Ultraviolet Laser Induced Periodic Surface Structures”, Clark and Emmony, Physical Review B, Vol. 40, No. 4, pgs 2031-2041 (1989); “Femtosecond Laser Induced Periodic Surface Structure on Diamond Film”, Wu, Ma, Fang, Liao, Yu, Chen, Wang, Applied Physics Letters, Vol. 82, No. 11, pgs 1703-1705 (2003); and “Self Organixed Nanogratings in Glass Irradiated by Ultrashort Light Pulses”, Shimotsuma, Kazansky, Qui, Hirao, Physical Review Letters, Vol. 19, No. 24, pgs 247205-1 to 4 (2003).